Protective film and method for preparing same

ABSTRACT

A high-quality protective film for a dry film resist is provided. A film of a polyethylene is used as the protective film, the polyethylene being prepared by pressurizing ethylene with use of an ultra-high pressure compressor and then polymerizing the ethylene at a reaction temperature of 190° to 300° C. and a reaction pressure of not lower than 167 MPa in the presence of a radical polymerization initiator, or by pressuring ethylene with use of an ultra-high pressure compressor and then polymerizing the ethylene at a reaction temperature of 190° to 300° C. in the presence of a radical polymerization initiator while allowing a radical polymerization inhibitor to be present in the reaction system.

This is a continuation of application Ser. No. 09/736,010 filed Dec. 13,2000 and now U.S. Pat. No. 6,664,346 issued Dec. 16, 2003.

FIELD OF THE INVENTION

The present invention relates to a protective film and a method forpreparing a polyethylene for the film. More particularly, the inventionis concerned with a protective film which is directly laminated to afilm resist to protect the film resist, as well as a method forpreparing a polyethylene for the film. The protective film is alsosometimes called a masking film or a cover film.

BACKGROUND OF THE INVENTION

A film resist (also called a dry film resist) for forming aphotosensitive layer used as an etching resist in the fabrication of alead frame or a printed circuit board which are used in mounting asemiconductor integrated circuit, is generally constructed such that apositive or negative photosensitive composition is formed on a supportfilm (a polyester film in many cases) and a protective film is laminatedthereon. As photosensitive compositions are known those of an alkalidevelopment type in which an unexposed area (negative type) or anexposed area (positive type) is removed with an aqueous alkali solutionand those of a solvent development type in which the said area isremoved with an organic solvent. The photosensitive composition used isa solid, but is not so hard as its shape does not change, having acertain degree of dimensional deformation.

As an example, a description will be given below of a method offabricating a printed circuit board or a lead frame in accordance with ametal etching process using an alkali development type film resist.

First, a protective film laminated directly onto a film resist is peeledoff and the film resist is laminated to a metallic substrate such as acopper-clad laminate so as to become direct contact with the substrateby means of pressure rolls. Then, a desired pattern is printed to thefilm resist on the metallic substrate by exposure through a mask withthe pattern plotted thereon.

By subsequent development using weak alkali water there is formed aresist pattern on the metallic substrate. The metallic substrate is thensubjected to etching with the resist pattern as a mask and thereafterthe resist pattern is peeled off using strong alkali water, whereby aprinted circuit board or a lead frame is fabricated.

In the process of laminating the film resist to the metallic substrateafter removal of the protective film, as pointed out in JP 11-153861A,there occur air voids in the interface between the metallic substrateand the film resist, and due to the presence of such air voids there mayoccur a pattern loss in forming the resist pattern or there may occur acircuit lead loss in etching the substrate. As mentioned also in theabove reference, the thinner the film resist as a photosensitive layer,the more likely the occurrence of such air voids.

In the above reference there is made judgment on the basis of the numberof air voids formed, but a more practical method involves forming aplotting pattern with a line width of several ten micrometers (μm) as asubstitute for circuit and checking defects on the pattern.

As the material of the protect film, polyesters such as polyethyleneterephthalate and polyolefins such as polypropylene and polyethylene areused in many cases, but in Comparative Example 1 described in the abovereference a protective film formed of polyethylene is an undesirableexample.

The protective film itself in the above reference is required to beimproved. More particularly, in the above reference, as a property ofthe protective film, the number of fish-eyes not smaller than 80 μm isdefined. In many cases, such large fish-eyes as are not smaller than 80μm in diameter result from incorporation of undissolved and deterioratedportions of the material into the film, as noted also in the abovereference.

However, in an effort to improve the protective film described in theabove reference, even if an attempt is made to fabricate such a film asis small in the number of fish-eyes of even a smaller diameter, say, 30μm or so, the cause of formation of such fine fish-eyes is not fullyclear yet. Besides, even the relation between such fine fish-eyes andthe formation of the foregoing air voids is not fully clear yet becauseof the very small size of the fish-eyes. In more particular terms, themeasurement of fish-eyes is based on observation of film transmittedlight, and unevenness of the film surface is not determined directly.Therefore, in the case of fish-eyes of a large diameter, there may be acorrelation thereof with unevenness of the film surface and the shapethereof, but in the case of fine fish-eyes 30 μm or so in diameter, thefish-eyes merely indicate the trace of resin flow and may not bear adirect relation to unevenness of the film surface. Thus, it is actuallydifficult to judge whether such fine fish-eyes are correlated or notwith unevenness of the film surface and the shape thereof. As a matterof course, it is difficult to judge whether the film permits theformation of the foregoing plotting pattern.

Further, the aforementioned adaptability as a protective film formed ofpolyethylene is difficult to be changed by changing the film formingmethod and film forming conditions. It is necessary to alter theessential properties of the polyethylene used. No matter how the filmforming method and forming conditions from polyethylene may be changed,there is a limit to the improvement of the aforementioned adaptabilityas a protective film. By merely changing the film forming method andforming conditions it will be difficult to obtain a useful protectivefilm. For example, even if polyethylene is merely filtered through afilter before or during film formation, no improvement is made in manycases. In the case of the foregoing large fish-eyes not smaller than 80μm in diameter there is recognized a certain decreasing effect in theirnumber by the adoption of physical removing means such as a filter, andthus it is presumed that there will be an essential difference from finefish-eyes 30 μm or so in diameter.

OBJECT OF THE INVENTION

It is an object of the present invention to solve the above-mentionedproblems. Particularly, it is an object of the invention to establish amethod for preparing a polyethylene superior as a dry film resistprotecting film for which a high-grade of protecting function isrequired and thereby provide a novel protective film, a method forfabricating the film, and a laminate using the film.

SUMMARY OF THE INVENTION

Having made, for achieving the above-mentioned object, earnest studiesabout a method for preparing a high pressure process low-densitypolyethylene and a polyethylene film formed using the polyethylene, thepresent inventors found out that a high reaction pressure and thepresence of a radical polymerization inhibitor in a reaction system arerelated to achieving the above-mentioned object, and succeeded ingreatly improving the suitability of the polyethylene film as aprotective film for a dry film resist.

More specifically, the present invention, in the first aspect thereof,resides in a protective film formed from a polyethylene, thepolyethylene being prepared by pressurizing ethylene with use of anultra-high pressure compressor and then polymerizing the ethylene at areaction temperature of 190° to 300° C. and a reaction pressure of notlower than 167 MPa in the presence of a radical polymerizationinitiator.

The present invention, in the second aspect thereof, resides in aprotective film formed from a polyethylene, the polyethylene beingprepared by pressurizing ethylene with use of an ultra-high pressurecompressor and then polymerizing the ethylene at a reaction temperatureof 190° to 300° C. in the presence of a radical polymerization initiatorwhile allowing a radical polymerization inhibitor to be present in thereaction system at a ratio such that the concentration of the radicalpolymerization inhibitor at an outlet of the ultra-high pressurecompressor is 5 to 1000 wt. ppm relative to the ethylene.

The present invention, in the third aspect thereof, resides in aprotective film according to the above second aspect of the invention,wherein the reaction pressure in the polymerization of ethylene is notlower than 167 MPa.

The present invention, in the fourth aspect thereof, resides in aprotective film according to the above second or third aspect of theinvention, wherein the radical polymerization inhibitor is a phenoliccompound or a quinone compound.

The present invention, in the fifth aspect thereof, resides in aprotective film according to any of the above first to fourth aspect ofthe invention, wherein the ethylene polymerization is performed using anagitation vessel type reactor and at a condition which gives anintra-reactor average residence time of 5 to 30 seconds.

The present invention, in the sixth aspect thereof, resides in aprotective film constituted by a film of a polyethylene, thepolyethylene having an MFR of 0.3 to 30 (g/10 min) and a density of0.913 to 0.930 (g/cm³) and, as a 30 μm thick film thereof, having a hazeof 1 to 50 (%) and containing not more than 40 fish eyes per 10 cm²which are not smaller than 30 μm and smaller than 0.20 mm in the majordiameter and not more than 1.0 fish-eye per 1.0 m² which is not smallerthan 0.20 mm in the major diameter.

The present invention, in the seventh aspect thereof, resides in alaminate comprising a film resist and the protective film mentioned inany of the above first to sixth aspects of the invention laminateddirectly to the film resist.

The present invention, in the eighth aspect thereof, resides in a methodfor preparing a polyethylene for a protective film, comprisingpressurizing ethylene by means of an ultra-high pressure compressor andthen polymerizing the ethylene at a reaction temperature of 190° to 300°C. and a reaction pressure of not lower than 167 MPa in the presence ofa polymerization initiator.

The present invention, in the ninth aspect thereof, resides in a methodfor preparing a polyethylene for a protective film, comprisingpressurizing ethylene by means of an ultra-high pressure compressor andthen polymerizing the ethylene at a reaction temperature of 190° to 300°C. in the presence of a polymerization initiator while allowing aradical polymerization inhibitor to be present in the reaction system ata ratio such that the concentration of the radical polymerizationinhibitor at an outlet of the ultra-high pressure compressor is 5 to1000 wt. ppm relative to ethylene.

The present invention, in the tenth aspect thereof, resides in a methodfor preparing a polyethylene according to the above ninth aspect of theinvention, wherein the ethylene polymerization is performed at areaction pressure of not lower than 167 MPa.

The present invention, in the eleventh aspect thereof, resides in amethod for preparing a polyethylene for a protective film according tothe above ninth or tenth aspect, wherein the polymerization inhibitor isa phenolic compound or a quinone compound.

The present invention, in the twelfth aspect thereof, resides in amethod for preparing a polyethylene for a protective film according toany of the above eighth to eleventh aspect of the invention, wherein theethylene polymerization is performed using an agitation vessel typereactor and at a condition which gives an intra-reactor averageresidence time of 5 to 30 seconds.

The present invention, in the thirteenth aspect thereof, resides in amethod for preparing a polyethylene for a protective film according toany of the above eighth to twelfth aspects, wherein the polyethylene hasan MFR of 0.3 to 30 (g/10 min) and a density of 0.913 to 0.930 (g/cm³)and, as a 30 μm film thereof, has a haze of 1 to 50 (%) and contains notmore than 40 fish-eyes per 10 cm² which are not smaller than 30 μm andsmaller than 0.20 mm in the major diameter and not more than 1.0fish-eye per 1.0 m² which is not smaller than 0.20 mm in the majordiameter.

The present invention, in the fourteenth aspect thereof, resides in amethod for preparing a laminate, comprising pressurizing ethylene bymeans of an ultra-high pressure compressor, then polymerizing theethylene at a reaction temperature of 190° to 300° C. and a reactionpressure of not lower than 167 MPa in the presence of a polymerizationinitiator to afford a polyethylene having an MFR of 0.3 to 30 (g/10 min)and a density of 0.913 to 0.930 (g/cm³), having a haze of 1 to 50% as a30 μm film thereof and containing not more than 40 fish-eyes per 10 cm²which are not smaller than 30 μm and smaller than 0.20 mm in the majordiameter and not more than 1.0 fish-eye per 1.0 m² which are not smallerthan 0.20 mm in the major diameter, and laminating the polyethylene filmdirectly as a protective film to a film resist.

The present invention, in the fifteenth aspect thereof, resides in amethod for preparing a laminate, comprising pressurizing ethylene bymeans of an ultra-high pressure compressor, then polymerizing theethylene at a reaction temperature of 190° to 300° C. in the presence ofa polymerization initiator while allowing a radical polymerizationinhibitor to be present in the reaction system at a ratio such that theconcentration of the radical polymerization inhibitor at an outlet ofthe ultra-high pressure compressor is 5 to 1000 wt. ppm relative toethylene to afford a polyethylene having an MFR of 0.3 to 30 (g/10 min)and a density of 0.913 to 0.930 (g/cm³), having a haze of 1 to 50% as a30 μm film thereof and containing not more than 40 fish-eyes per 10 cm²which are not smaller than 30 μm and smaller than 0.20 mm in the majordiameter and not more than 1.0 fish-eye per 1.0 m² which is not smallerthan 0.20 mm in the major diameter, and laminating the polyethylene filmdirectly as a protective film to a film resist.

The present invention, in the sixteenth aspect thereof, resides in amethod for preparing a laminate, comprising pressurizing ethylene bymeans of an ultra-high pressure compressor, then polymerizing theethylene at a reaction temperature of 190° to 300° C. and a reactionpressure of not lower than 167 MPa in the presence of a polymerizationinitiator while allowing a radical polymerization inhibitor to bepresent in the reaction system at a ratio such that the concentration ofthe radical polymerization inhibitor at an outlet of the ultra-highpressure compressor is 5 to 1000 wt. ppm relative to ethylene to afforda polyethylene having an MFR of 0.3 to 30 (g/10 min) and a density of0.913 to 0.930 (g/cm³), having a haze of 1 to 50% as a 30 μm filmthereof and containing not more than 40 fish-eyes per 10 cm² which arenot smaller than 30 μm and smaller than 0.20 mm in the major diameterand not more than 1.0 fish-eye per 1.0 m² which is not smaller than 0.20mm in the major diameter, and laminating the polyethylene film directlyas a protective film to a film resist.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail hereinunder.

The polyethylene used for forming the protective film of the presentinvention is prepared by a high-pressure radical polymerization ofethylene using, for example, an organic peroxide or oxygen as a radicalpolymerization initiator.

The high-pressure radical polymerization process generally comprisespressurizing ethylene by means of an ultra-high pressure compressor,then introducing the ethylene into a tubular reactor or an agitationvessel type reactor, allowing the ethylene to be polymerized in thepresence of a radical polymerization initiator, and subsequentlyreleasing the pressure to afford a polyethylene.

The radical polymerization initiator is a free radical generatingcompound, examples of which include oxygen and organic peroxides such ast-butylperoxy pivalate, t-butylperoxy octoate, t-butylperoxy acetate,and t-butylperoxy benzoate. The radical polymerization initiator ispoured into the reactor by a conventional method.

The pressurization of ethylene is effected usually by a two-stepcompression using a high-pressure compressor and an ultra-high pressurecompressor. It is the high-pressure compressor that pressurizes ethyleneup to about 10˜40 MPa and it is the ultra-high pressure compressor thatfurther pressurizes ethylene up to the reaction pressure. The ethyleneleaving the ultra-high pressure compressor is introduced into thereactor and is polymerized therein. As the reactor there may be adoptedeither the foregoing tubular reactor or agitation vessel type reactor,the latter being preferred.

The reaction temperature in the ethylene polymerization is in the rangeof 190° to 300° C., preferably 200° to 280° C., more preferably 210° to265° C. If the reaction temperature is outside the above range, apolymer-to-polymer crosslinking reaction is accelerated and the fish-eyedensity increases. A lower limit value of the polymerization reactiontemperature represents the temperature of a portion where thepolymerization proceeds and a relatively clear rise of temperature isrecognized downstream of the portion where ethylene gas and thepolymerization initiator contact each other for the first time withinthe reactor, while an upper limit value of the polymerization reactiontemperature represents the maximum temperature in the reactor.

A suitable type and amount of a polar monomer, e.g., vinyl acetate or(meth)acrylic acid ester, may be copolymerized with ethylene insofar asthe performance as a protective film is not impeded.

In the ethylene polymerization according to the present invention it isimportant that the polymerization be performed at a certain reactionpressure or higher. The reaction pressure means the lowest pressure inthe reactor; more specifically, it is not lower than 167 MPa, preferablynot lower than 190 MPa. If the reaction pressure is lower than 167 MPa,there will arise a region in the reactor in which region there occursphase separation between part of the ethylene and a concentratedpolyethylene phase. As a result, the polyethylene-to-polyethylenecontact probability increases in the concentrated polyethylene phase andit becomes easier for the polyethylene crosslinking reaction to takeplace. Thus, at a pressure lower than 167 MPa, it is difficult todiminish fine fish-eyes. If the reaction pressure is not lower than 167MPa, the interior of the reactor is maintained at a uniform phase, sothat the crosslinking reaction is difficult to occur. As to thepolymerization pressure, the higher, the better. But a substantial upperlimit of the reaction pressure is 393 MPa or so because a limit isencountered in the withstand pressure of various devices disposed in theprocess.

The feed rate of the reaction feedstock is controlled so that anintra-reactor average residence time in the polymerization is in therange of 5 to 30 seconds, preferably 8 to 25 seconds, more preferably 10to 18 seconds. If the average residence time is shorter than 5 seconds,unreacted radical polymerization initiator will flow out downward of thereactor, thus causing reaction to be started within a pipe not held in asatisfactory state of agitation or in a separator. If the initiatordecomposes in an unsatisfactory state of dispersion, the polymer will becrosslinked locally and form fish-eyes, which may lead to the occurrenceof an abnormal reaction. If the average residence time is longer than 30seconds, the crosslinking will be accelerated because the polymerproduced is exposed a long time to a free radical-containingenvironment. The “intra-reactor average residence time” stands for avalue obtained by dividing an internal volume (m³) of the reactor by avolume flow rate (m³/sec) of the total ethylene gas entering the reactor

In the ethylene polymrization according to the present invention, it ispreferable that a polymerization inhibitor be present within thereactor, in addition to setting the reaction pressure at 167 MPa orhigher. By combining these two means it is possible to synergisticallyattain the reduction of fish-eyes, especially fine fish-eyes.

The amount of the polymerization inhibitor added into the reactionsystem is in the range of 5 to 1000 wt. ppm, preferably 50 to 500 wt.ppm, more preferably 100 to 300 wt. ppm, relative to ethylene. It goeswithout saying that a suitable amount thereof should be selected so asnot to impede the polymerization of ethylene monomer itself. Thoughsomewhat different depending on the degree of the polymerizationinhibiting effect of the polymerization inhibitor used, a smaller amountof the polymerization inhibitor than the lower limit will not attain asatisfactory fish-eye reducing effect, and a larger amount thereof thanthe upper limit is not practical because the fish-eye reducing effectwill reach the ceiling and the amount of the polymerization initiatorconsumed will increase to a great extent. As will be noted later, sinceunreacted ethylene is recycled and reused, the flow rate thereof becomesmaximum at the outlet of the ultra-high pressure compressor. Thepolymerization inhibitor is made present in the reaction system at sucha ratio as to give the above concentration thereof at the said outlet.

The polymerization inhibitor may be added at any suitable position aslong as it is present in the reaction system. However, in the case wherethe polymerization inhibitor is added at a rear position with respect tothe outlet of the ultra-high pressure compressor, it is necessary to usean ultra-high pressure pump for the addition of the polymerizationinhibitor. Thus, it is preferred that the polymerization inhibitor beadded at a front position with respect to the ultra-high pressurecompressor. More specifically, the position where the polymerizationinhibitor is to be added is before the inlet of the high-pressurecompressor or downstream of the outlet of the high-pressure compressorand upstream of the inlet of the ultra-high pressure compressor. It isoptional whether the polymerization inhibitor is to be added alone intothe reaction system or to be added as a mixture thereof with a molecularweight modifier.

The polymerization inhibitor to be present in the reaction system is notspecially limited if only it is an ethylene radical polymerizationinhibitor. Examples are phenolic compounds and quinone compounds,including hydroquinone, monomethyl ether hydroquinone,2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol,3,5-bis(1,1-dimethyl ethyl)-4-hydroxybenzenepropionic acid octadecylester, thiobisphenol, quinone, and 4-t-butylpyrocatechol. Particularlypreferred are monomethyl ether hydroquinone,2,6-di-t-butyl-4-methylphenol, and 2,6-di-t-butyl-4-ethylphenol, whichare relatively low in molecular weight, not too large in polarity, andare superior in dispersibility into ethylene. More preferred are2,6-di-t-butyl-4-methylphenol and 2,6-di-ti-butyl-4-ethylphenol which donot inhibit the polymerization reaction to an excess degree. Thesepolymerization inhibitors may be used each alone or as a combination oftwo or more. The polymerization inhibitor used may be added as asolution in a solvent into the reaction system. As examples of thesolvent are mentioned saturated hydrocarbons having 6 to 14 carbonatoms. As the case may be, two or more of these solvents may be combinedand used. Preferred solvents are those which are good solvents for thepolymerization inhibitor used and which do not exert any bad influenceon the polymerization system. Further, it is preferable to select such asolvent as remains in as small an amount as possible in the polymer,taking into account its boiling point and separation efficiency from thepolymer.

In carrying out the polymerization there may be used a molecular weightmodifier as necessary. As the molecular weight modifier there may beused any of various chain transfer agents. As examples of chain transferagents are mentioned olefin compounds such as propylene, butene, andhexene, paraffins such as ethane, propane, and butane, and aromatichydrocarbons such as toluene, xylene, and ethylbenzene.

If pressure is released after the end of the polymerization, there isobtained a desired polyethylene, e.g., a polyethylene having an MFR [JISK 6760 (temperature: 190° C., load: 2.16 kg) of 0.3 to 30 (g/10 min) anda density of (JIS K 6760) of 0.913 to 0.930 (g/cm³). Unreacted ethylenewhich is recovered upon release of pressure can be recycled and reusedafter being pressurized again. It suffices for the polyethylene thusobtained to have an MFR of 0.3 to 30, preferably 1 to 10, morepreferably 2 to 5. If its MFR is outside this range, it will becomedifficult to form a film of the polyethylene stably. Likewise, itsuffices for the polyethylene to have a density of 0.913 to 0.930,preferably 0.918 to 0.930, more preferably 0.920 to 0.928. If thedensity of the polyethylene is lower than 0.913, the stiffness of theresulting film will be lowered and the working efficiency may become toolow, while if the polyethylene density exceeds 0.930, the flexibility ofthe resulting film will be deteriorated and there may arise a problem inpoint of close adhesion of the film when used as a protective film.

As a method for forming a film to be used as a protective film there maybe adopted a known film forming method. For example, there may beadopted an inflation method (air- or water-cooling) or a T-die method.As the case may be there may be added a stretching process such asuniaxial stretching or biaxial stretching. The film forming temperatureand haul-off speed are not specially limited, but are suitably in therange of 130° to 230° C. and in the range of 5 to 40 m/min,respectively. A suitable film thickness is selected, but as a protectivefilm the film thickness is preferably in the range of 10 to 100 μm, morepreferably 15 to 50 μm. The polyethylene which constitutes theprotective film of the present invention may be blended with apolyolefin resin such as a liner low-density polyethylene, ahigh-density polyethylene, an ethylene-propylene copolymer, or anethylene-butene copolymer, insofar as the conditions defined forfish-eye, etc. in the invention are not badly influenced. Whererequired, additives usually employed in the preparation of polyolefinssuch as antioxidant, anti-blocking agent, lubricant, antistatic agent,and ultraviolet ray absorber.

When the polyethylene is prepared in the presence of a radicalpolymerization inhibitor in the polymerization reaction system, acertain amount of the radical polymerization inhibitor remains in theresulting product. This residual radical polymerization inhibitor alsopossesses an oxidation preventing ability, and if the concentrationthereof is sufficient, the addition of an antioxidant into thepolyethylene product may be omitted.

The protective film of the present invention formed from thepolyethylene thus prepared has a haze (determined in accordance with JISK 7105) of 1 to 50 (%) and contains not more than 40 fish-eyes per 10cm² which are not smaller than 30 μm and smaller than 0.20 mm in themajor diameter and not more than 1.0 fish-eye per 1.0 m² which are notsmaller than 0.20 mm in the major diameter.

The “fish-eye” means an optically non-uniform region observed from filmtransmitted light. Fish-eyes of a large diameter, say, 80 μm or so, areforeign matters in many cases. However, fish-eyes of a very smalldiameter, say, 30 μm or so, have not been made fully clear yet becauseof a very small diameter thereof. Thus, the cause of formation of suchfine fish-eyes is uncertain and a satisfactory cure has not beendeveloped yet. Besides, fine fish-eyes as an optically non-uniformregion observed from transmitted light are not presumed to be directlycorrelated with the generation of air voids.

Fish-eyes (large fish-eyes) not smaller than 0.20 mm in terms of themajor diameter are mainly caused by an oxidatively deteriorated resin.It is preferable that there be no such large fish-eyes because theywould exert a serious influence on the film smoothness. The number ofsuch large fish-eyes should be not larger than 1.0 pc./1.0 m²,preferably not larger than 0.5 pc./1.0 m², more preferably not largerthan 0.3 pc./1.0 m². An upper limit of the major diameter of such largefish-eyes should be 1.5 mm or so from the standpoint of preventingserious poor appearance and poor close adhesion.

On the other hand, as to fish-eyes smaller than 0.20 mm in terms of themajor diameter, the number of fish-eyes (small fish-eyes) not smallerthan 30 μm and smaller than 0.20 mm in the major diameter is not largerthan 40 per 10 cm² preferably not larger than 10 per 10 cm², morepreferably not larger than 2 per 10 cm². A lower limit of the number ofsmall fish-eyes is usually 0.05 pc./10 cm² from the standpoint ofmanagement of film manufacturing conditions. The above range of thenumber of small fish-eyes substantially corresponds to 5 pc./m² or lessas the number of fish-eyes not smaller than 80 μm in diameter. It isrecognized that as the fish-eye diameter decreases, the number offish-eyes increases progressively. Therefore, even if the number of 80μm fish-eyes is zero, a considerable number of fish-eyes 30 μm or so indiameter are usually present (even if it is taken into account that themeasurement of such fine fish-eyes as 30 μm is difficult).

The density of fish-eyes in the film of the invention is determined froma 30 μm thick film formed under the conditions of set extrudertemperatures of 130–150° C. in a cylinder portion, 150° C. in an adapterdie portion, a haul-off speed of 20 m/min, and a blow-up ratio of 2.0,using an air-cooled type inflation film molding machine (extruder barreldia. 45 mm, die dia. 80 mm, a product of Modern Machinery Co.), within asimple clean room (cleanness: Federal Standard 209D Class 10000).

Fish-eyes are measured in the following manner.

Large fish-eyes not smaller than 0.20 mm in the major diameter aremeasured by in-line measurement using a laser counter or a CCD cameraattached to a film forming machine, while small fish-eyes not smallerthan 30 μm and smaller than 0.20 mm are measured in accordance with amethod wherein an image in transparent mode is subjected to off-lineanalysis using a CCD scanner with a resolution of 1600 dpi. Themeasurement area is not smaller than 30 cm² in the measurement of smallfish-eyes not smaller than 30 μm and smaller than 0.20 mm in terms ofthe major diameter and is not smaller than 100 m² in the measurement oflarge fish-eyes not smaller than 0.20 mm in the major diameter. Therespective actually counted numbers are converted to numbers per unitarea.

An example of use of the protective film according to the presentinvention will now be described. As already explained, a dry film resisthas a construction such that a resist layer formed of a photosensitiveresin is formed on a base film of polyethylene terephthalate forexample, and a polyethylene film as a protective film is laminated to anupper surface of the resist layer so as to be in direct contact with theresist layer. As the resist there may be used any of known resistsemployable as dry film resists. The thinner the film resist layer, thebetter, because the resolution is improved. Particularly, from thestandpoint of resolution, a thinner resist layer has been demandedrecently. The protective film of the present invention is suitable forlamination to a resist layer not larger than 30 μm, preferably notlarger than 25 μm, in thickness. A lower limit value of thickness is notspecially limited, but is usually not smaller than 0.1 μm, preferablynot smaller than 1 μm.

The method of forming a resist film on a base film and subsequentlylaminating a polyethylene film to the resist film can be carried out ina known manner.

The following is an outline of a resist process using the laminate offilm resist and polyethylene film thus obtained, more particularly, thelaminate of protective film/film resist/base film.

First, as the film resist there may be used any of known film resists.For example, it can be such a film resist as is exemplified in theforegoing JP 11-153861A, which film resist is formed from aphotosensitive composition containing (a) a binder polymer prepared bycopolymerizing acrylic acid or methacrylic acid and an alkyl esterthereof as constituent monomers, (b) a monomer containing at least onephotopolymerizable ethylenically unsaturated group in the molecule, and(c) a photopolymerization initiator. These ingredients may becommercially available ones.

As the base film (support film) there may be used a polyester film,e.g., a polyethylene terephthalate film. The thickness of the base filmis not specially limited, but is usually selected from the range of 1 to30 μm. The base film may also be a commercially available one.

The resist layer in the above laminate of protective film/filmresist/base film is affixed to one metallic side of a metallic substrateso that the base film becomes the top layer while peeling off theprotective film. That is, the resist layer is affixed to one metallicside of the metallic substrate so that the resist surface after removalof the protective film comes into contact with the metallic side. Themetallic side of the substrate is polished beforehand suitably. Affixingof the resist layer to one metallic side of the substrate may be doneusing pressure rolls if necessary. The protective film which has beenpeeled off can be discarded or recycled. A polyethylene prepared by ahigh-pressure radical polymerization process comes into close contactwith the resist surface, but the adhesion thereof to the resist is nothigh, in other words, its releasability is good. Thus, when theprotective film is peeled off, there is little fear of the resistremaining on the protective film.

Next, ultraviolet ray or any other active ray is radiated to the filmresist through a photomask having a desired pattern, causing part of theresist layer to be photocured or photodecomposed correspondingly to thepattern, while allowing the residual resist layer to remain unchanged.Thereafter, in the case of a positive resist, the uncured resist layerportion is dissolved off using a developer such as an organic solvent oran aqueous alkali solution, while allowing the photocured resist layerportion to remain as It is. In this way a cured resist layer having adesired pattern can be formed on the substrate. Where required, etchingis performed subsequently to form a circuitry, etc. on the metallicsubstrate.

The protective film of the present invention exerts no physical andchemical influence on the film resist surface with which it comes intodirect contact. Besides, air voids are scarcely formed and consequentlythere scarcely arises a cured resist layer having a pattern loss at thetime of pattern formation. Thus, the protective film of the invention issuitable as a protective film which comes into direct contact with theresist film. Further, as shown in the foregoing reference, the thinnerthe resist film, the easier the occurrence of such a defect as patternloss, but in the case of the protective film according to the presentinvention, even when laminated to a dry film resist as thin as 30 μm orless, preferably 1 to 25 μm, there are few air voids formed andconsequently there is little fear that a cured resist layer having apattern loss may occur at the time of pattern formation.

EXAMPLES

The present invention will be described below by way of examples.

Example 1

Using a high-pressure process low-density polyethylene manufacturingequipment having an agitation vessel type reactor, under the conditionsof reaction temperatures ranging from 210° C. in a lowest temperatureportion to 265° C. in a highest temperature portion, a reaction pressureof 196 MPa, and an intra-reactor average residence time of 17 seconds, asolution of 2,6-di-t-butyl-4-methylphenol in isoparaffin (concentration:150 g/l) was fed at a position downstream of an outlet of ahigh-pressure compressor and upstream of an inlet of an ultra-highpressure compressor in such a manner that the concentration of2,6-di-t-butyl-4-methylphenol in ethylene at an outlet of the ultra-highpressure compressor is 170 wt. ppm, to afford a polyethylene.

Prior to preparation of the object product, a similar product almostequal in MFR and density was produced over a period of not shorter than8 hours to purge the interior of the process system thoroughly. For theportions present in the process from after resin pelletization tobagging, cleaning was performed to a thorough extent so as not to permitincorporation of residual polyethylene different in molecular weight,and also during bagging there were given dust-proof considerations asmuch as possible to prevent the entry of dust suspended in the air.

As a result, there was produced a lot of about 40T products. Theproducts have uniform physical properties and quality because within thelot there is performed a homogenizing operation by dry blending.

Using the polyethylene thus prepared and while paying attention to theprevention of dust entry in the handling process and also topre-cleaning of residues in a molding machine, a 30 μm thick film wasformed in a simple clean room of cleanness class 10000 (Federal Standard209D), by means of an air-cooled type inflation film molding machine(extruder barrel dia. 45 mm, full-flighted screw, mounted filter 120mesh, die dia. 80 mm, a product of Modern Machinery Co.), under theconditions of set extruder temperatures of 130˜150° C. in a cylinderportion, 150° C. in an adapter die portion, a haul-off speed of 20m/min, and a blow-up ratio of 2.0.

As to the density of large fish-eyes not smaller than 0.20 mm in themajor diameter, the number thereof was measured in-line during moldingby means of a laser fish-eye counter (Laser Eye Model TPLS-J04T, aproduct of Yasukawa Electric Mfg. Co.) and was then converted to thenumber per predetermined unit area. A single measurement covered an areaof 320 m². This measurement was conducted three times for each productlot and a mean value was calculated.

As to the density of small fish-eyes not smaller than 30 μm and smallerthan 0.20 mm, the film obtained was cut into a size approximately equalto A4 size and an arbitrary portion was scanned in a transparent modewith a resolution of 1600 dpi using a CCD scanner (EPSON ES-2000),followed by analysis of the resulting image and subsequent conversioninto the number per predetermined unit area. A single measurementcovered an area of 33.3 cm². The measurement was made a total of threetimes and a mean value was calculated. The results of the fish-eyedensity measurements are shown in Table 1.

The polyethylene film was used as a protective film for a dry filmresist and was found to be fully employable for a plotting pattern witha minimum stroke width of 20 μm.

The formation and judgment of the above plotting pattern were made inthe following manner (this method was applied also to Examples andComparative Examples which follow).

The polyethylene film (30 μm thick) was laminated as a protective filmonto the resist in a laminate of 16 μm polyethylene terephthalate film(support film)/commercial film resist (20 μm).

This three-layer film laminate was laminated to a copper-clad laminatewhile peeling off the protective film. Thereafter, exposure,development, and etching were conducted by conventional methods using aline drawing pattern mask with a minimum stroke width of 20 μm to form aline drawing on the copper-clad laminate. The adaptability to the linedrawing pattern was determined by observing the line drawing through amicroscope.

Example 2

Using the same conditions and method as in Example 1 except that thesolution of 2,6-di-t-butyl-4-methylphenol in isoparaffin was not chargedinto the reaction system, a polyethylene was prepared (about 80T) andfilm was formed from the polyethylene, followed by measurement. Theresults of fish-eye density measurements are shown in Table 1.

The polyethylene film thus formed was used as a protective film for adry film resist and was found to be fully employable for a line drawingpattern with a minimum stroke width of 30 μm. However, there wasrecognized a partial inconvenience at a minimum stroke width of 20 μm.

Example 3

Using the same conditions and method as in Example 1 except that thereaction pressure was 173 MPa and the solution of2,6-di-t-butyl-4-methylphenol in isoparaffin was not charged into thereaction system, a polyethylene was prepared (about 80T) and film wasformed from the polyethylene, followed by measurement. The results offish-eye density measurements are shown in Table 1.

The polyethylene film thus formed was used as a protective film for adry film resist and was found to be employable for a line drawingpattern with a minimum stroke width of 75 μm. It could be used suitablyas a general-purpose protective film for an acrylic sheet or the like(also in the following).

Example 4

Using a high-pressure process low-density polyethylene manufacturingequipment having a tubular type reactor and under the conditions ofreaction temperatures ranging from 195° C. in a lowest temperatureportion to 275° C. in a highest temperature portion and a reactionpressure of 240 MPa, a polyethylene was prepared (about 50T) in the sameprocedure as in Example 1, followed by film formation and measurementusing the same conditions and methods as in Example 1. The results offish-eye density measurements are shown in Table 1.

The polyethylene film thus formed was used as a protective film for adry film resist and was found to be employable for a line drawingpattern with a stroke width of 75 μm. It could be used suitably as ageneral-purpose protective film.

Comparative Example 1

Polyethylene preparation, film formation, and fish-eye measurement wereconducted in the same way as in Example 3 except that the reactionpressure was set at 150 MPa. The results of fish-eye densitymeasurements are shown in Table 1.

When the polyethylene film thus formed was used as a protective film fora dry film resist, there partially occurred inconvenience even in alow-grade application of 100 μm or more in terms of a minimum strokewidth.

Comparative Example 2

Polyethylene preparation (about 40T), film formation, and fish-eyemeasurement were conducted in the same way as in Example 3 except thatthe reaction temperature range was set at 175° C. in a lowesttemperature portion to 305° C. in a highest temperature portion. Theresults of fish-eye density measurements are shown in Table 1.

When the polyethylene film thus formed was used as a protective film fora dry film resist, there partially occurred inconvenience even in alow-grade use of 100 μm or more in terms of a minimum stroke width.

Comparative Example 3

Polyethylene preparation (about 30T), film formation, and fish-eyemeasurement were conducted in the same way as in Example 3 except thatthe intra-reactor average residence time was set at 34 seconds. Theresults of fish-eye density measurements are shown in Table 1.

When the polyethylene film thus formed was used as a protective film fora dry film resist, it was found to be employable for a plotting patternwith a minimum stroke width of 100 μm or more. Further, it could be usedsuitably as a general-purpose protective film.

TABLE 1 Fish-eye 80 μm or more and smaller MFR Fish-eye 0.20 mm than0.20 mm in g/10 Density Haze or more in major major dia., min g/cm³ %dia., pc/1.0 m² pc/10 cm² Example 1 3.1 0.925 11.1 0.25 0.5 Example 23.6 0.925 13.2 0.37 7.8 Example 3 3.5 0.924 16.7 0.26 38.2 Example 4 3.00.926 9.7 0.87 37.5 Compara- 3.5 0.923 25.1 1.51 132 tive Example 1Compara- 3.3 0.923 30.1 3.26 522 tive Example 2 Compara- 3.3 0.921 15.10.51 117 tive Example 3 *In Examples 1 to 4 and Comparative Example 3the number of fish-eyes not smaller than 80 μm in the major diameter wasnot larger than 5 pc./m².

The presence and the number of fish-eyes not larger than 30 μm aredifficult to change even by changing film forming method and conditions.It is necessary to alter the essential properties of the polyethyleneused. This is because the cause of formation of such fine fish-eyes asnot larger than 30 μm has not been made clear yet.

In the present invention, by polymerizing ethylene at a high pressurewhich permits polymerization in a homogeneous phase and by making apolymerization inhibitor present in the reaction system, it is possibleto provide an effective and superior polyethylene film as a protectivefilm even for a thin resist layer.

Thus, it turns out that the protective film according to the presentinvention is fully employable even for a plotting pattern with a minimumstroke width of 20 μm.

Further, a low-density polyethylene prepared according to ahigh-pressure radical polymerization method is soft and so is superiorin its close adhesion to a film resist surface and is thereforeconvenient for exhibiting a protecting function. Besides, since thereleasability of such a low-density polyethylene from a film resist isgood to a moderate extent despite its superior close adhesion, it can belaminated easily to a metallic substrate.

Thus, the polyethylene film prepared according to the present inventionis suitable as a protective film.

1. A method for preparing a polyethylene for a protective film whichcomprises pressurizing ethylene by means of an ultra-high pressurecompressor and then polymerizing the ethylene at a reaction temperatureof 90° to 300° C. and a reaction pressure of not lower than 167 MPa inthe presence of a polymerization initiator, wherein the polyethyleneprepared has an MFR of 0.3 to 30 (g/10 min) and a density of 0.913 to0.930 (g/cm³) and, as a 30 μm thick film thereof, has a haze of 1 to 50(%) and contains not more than 40 fish-eyes per 10 cm² which fish-eyesare not smaller than 30 μm and smaller than 0.20 mm in the majordiameter and not more than 1.0 fish-eye per 1.0 m² which fish-eye is notsmaller than 0.20 mm in the major diameter.
 2. A method for preparing alaminate which method comprises pressurizing ethylene by means of anultra-high pressure compressor, then polymerizing the ethylene at areaction temperature of 190° to 300° C. and a reaction pressure of notlower than 167 MPa in the presence of a polymerization initiator toafford a polyethylene having an MFR of 0.3 to 30 (g/10min) and a densityof 0.913 to 0.930 (g/cm³), having a haze of 1 to 50% as a 30 μm filmthereof and containing not more than 40 fish-eyes per 10 cm² whichfish-eyes are not smaller than 30 μtm and smaller than 0.20 mm in themajor diameter and not more than 1.0 fish-eye per 1.0 m² which fish-eyeis not smaller than 0.20 mm in the major diameter, and laminating thepolyethylene film directly as a protective film to a film resist.
 3. Amethod for preparing a laminate which method comprises pressurizingethylene by means of an ultra-high pressure compressor, thenpolymerizing the ethylene at a reaction temperature of 190° to 300° C.in the presence of a polymerization initiator while allowing a radicalpolymerization inhibitor to be present in the reaction system at a ratiosuch that the concentration of the radical polymerization inhibitor atan outlet of said ultra-high pressure compressor is 5 to 1000 wt. ppmrelative to ethylene to afford a polyethylene having an MFR of 0.3 to 30(g/10min) and a density of 0.913 to 0.930 (g/cm³), having a haze of 1 to50% as a 30 μm film thereof and containing not more than 40 fish-eyesper 10 cm² which fish eyes are not smaller than 30 μm and smaller than0.20 mm in the major diameter and not more than 1.0 fish-eye per 1.0 m²which fish-eye is not smaller than 0.20 mm in the major diameter, andlaminating the polyethylene film directly as a protective film to a filmresist.
 4. A method for preparing a laminate which method comprisespressuring ethylene by means of an ultra-high pressure compressor, thenpolymerizing the ethylene at a reaction temperature of 190° to 300° C.and a reaction pressure of not lower than 167 MPa in the presence of apolymerization initiator while allowing a radical polymerizationinhibitor to be present in the reaction system at a ratio such that theconcentration of the radical polymerization inhibitor at an outlet ofsaid ultra-high pressure compressor is 5 to 1000 wt. ppm relative toethylene to afford a polyethylene having an MFR of 0.3 to 30 (g/10 min)and a density of 0.913 to 0.930 (g/cm³), having a haze of 1 to 50% as a30 μm film thereof and containing not more than 40 fish-eyes per 10 cm²which fish eyes are not smaller than 30 μm and smaller than 0.20 mm inthe major diameter and not more than 1.0 fish eye per 1.0 m² whichfish-eye is not smaller than 0.20 mm in the major diameter, andlaminating the polyethylene film directly as a protective film to a filmresist.
 5. A method for preparing a polyethylene for a protective film,which comprises pressurizing ethylene by means of an ultra-high pressurecompressor and then polymerizing the ethylene at a reaction temperatureof 190° to 300° C. in the presence of a polymerization initiator whileallowing a radical polymerization inhibitor to be present in thereaction system at a ratio such that the concentration of the radicalpolymerization inhibitor at an outlet of said ultra-high pressurecompressor is 5 to 1000 wt. ppm relative to ethylene, wherein thepolyethylene prepared has an MFR of 0.3 to 30 (g/10 min) and a densityof 0.913 to 0.930 (g/cm³) and, as a 30 micron thick film thereof, has ahaze of 1 to 50% and contains not more than 40 fish-eyes per 10 cm²which fish-eyes are not smaller than 30 microns and smaller than 0.20 mmin the major diameter and not more than 1.0 fish-eye per 1.0m² whichfish-eye is not smaller than 0.20 mm in the major diameter.
 6. Method offorming a laminate comprising a protective film formed from apolyethylene and comprising the steps of pressurizing ethylene by meansof an ultra-high pressure compressor and then polymerizing the ethyleneat a reaction temperature of 190° to 300° C. and a reaction pressure ofnot lower than 167 MPa in the presence of a polymerization initiator,separately forming the protective film apart from a film resist,laminating the thus-formed protective film to the film resist, andpeeling off the protective film from the film resist to allow the filmresist to be further treated.
 7. Method of forming a laminate comprisinga protective film formed from a polyethylene and comprising the steps ofpressurizing ethylene by means of an ultra-high pressure compressor andthen polymerizing the ethylene at a reaction temperature of 190° to 300°C. in the presence of a polymerization initiator while allowing aradical polymerization inhibitor to be present in the reaction system ata ratio such that the concentration of the radical polymerizationinhibitor at an outlet of said ultra-high pressure compressor is 5 to1000 wt. ppm relative to ethylene, separately forming the protectivefilm apart from a film resist, laminating the thus-formed protectivefilm to the film resist, and peeling off the protective film from thefilm resist to allow the film resist to be further treated.
 8. Method offorming a laminate comprising a protective film formed from apolyethylene, said polyethylene having an MFR of 0.3 to 30 (g/10 min)and a density of 0.913 to 0.930 (g/cm³) and, as a 30 micron thick filmthereof, having a haze of 1 to 50 (%) and containing not more than 40fish-eyes per 10 cm² which fish-eyes are not smaller than 30 microns andsmaller than 0.20 mm in the major diameter and not more than 1.0fish-eye per 1.0 m² which fish-eyes are not smaller than 0.20 mm in themajor diameter, comprising the steps of separately forming theprotective film apart from a film resist, laminating the thus-formedprotective film to the film resist, and peeling off the protective filmfrom the film resist to allow the film resist to be further treated.